A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
During and after various steps in the production of devices using lithographic methods, it is necessary to make measurements on the devices that have been produced to assess whether the production process has proceeded correctly. Such measurements are collectively referred to as metrology. In view of the scale of devices produced by lithography, metrology methods generally involve illuminating the device structure or a test structure with a measurement beam and detecting the returning radiation. One such method is scatterometry, in particular angle-resolved scatterometry in which the spectrum of the radiation reflected by the structure under inspection is measured at different angles using a sensor such as a CCD in the pupil plane of an objective lens.
The sensitivity and speed of operation of most metrology devices is limited by the radiation source used to generate the measurement beam. Presently available sources include: Xe lamps, which have a good, wide bandwidth but poor luminance and poor spatial coherence; SLED lasers, which have good luminance and spatial coherence but not a particularly wide bandwidth and are not capable of generating wavelengths below about 400 nm.